Half of American adults have diabetes or prediabetes, characterized by abnormally high blood-sugar levels, a new study has found.

According to The Associated Press, 12 to 14 percent of U.S. adults had diagnosed diabetes, mostly of the Type 2 variety, which is linked with inactivity and obesity. At the same time, nearly 40 percent have prediabetes, or elevated blood sugar levels that can lead to Type 2 diabetes.

The study, published Tuesday in the Journal of the American Medical Association, also found that likely one-third of people with diabetes don't know they have it, as they've never received a diagnosis. This has prompted the medical establishment to increase its vigilance for the disease.

"By learning more about who has diabetes — and who has the disease but does not know it — we can better target research and prevention efforts," said Dr. Griffin Rodgers, director of the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), according to Yahoo News. "We have treatments to help people with diabetes, but treatments can only help those who have been diagnosed."

Significantly, half of Asian-Americans with diabetes did not know they had it. Hispanics, blacks, and Asian-Americans had higher rates of diabetes than whites.

Those with Type 2 diabetes are at increased risk of heart disease and stroke, as well as nerve damage, kidney failure, and blindness.

Researchers were hopeful in some of the data they found, including an indication that the prevalence of diabetes has changed little from 2007 to 2012 — suggesting a leveling off of the epidemic.

Overall, the new study examined surveys and blood samples from over 26,000 U.S. adults from 1988 to 2012.